Photo-Energized MoS2/CNT Cathode for High-Performance Li–CO2 Batteries in a Wide-Temperature Range

Highlights The unique layered structure and excellent photoelectric properties of MoS2 facilitate the abundant generation and rapid transfer of photo-excited carriers, which accelerate the CO2 reduction and Li2CO3 decomposition upon illumination. MoS2-based photo-energized Li–CO2 battery displays ultra-low charge voltage of 3.27 V, high energy efficiency of 90.2%, superior cycling stability after 120 cycles and high rate capability. The low-temperature Li–CO2 battery achieves an ultra-low charge voltage of 3.4 V at –30 °C with a round-trip efficiency of 86.6%. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01506-1.


Introduction
The rechargeable Li-CO 2 battery emerges as a newly conceptual and promising energy conversion and storage device to alleviate the environmental crisis and energy crisis, which can convert carbon dioxide into sustainable electricity with a standout theoretical specific capacity of 1876 Wh kg -1 [1][2][3][4][5][6][7][8][9].However, in spite of the above-mentioned favorable factors and promising prospects, the development of Li-CO 2 battery has been plagued by high voltage gap and slow kinetics of decomposition during charging due to the insulated discharge product Li 2 CO 3 with high thermodynamic stability [10][11][12].In recent years, some advances have been made for Li-CO 2 batteries with various catalysts including metal, alloy, single atom, and oxide, but their improved voltage gaps were still beyond 1 V and the challenging problem of high overpotential still exists to be addressed [13][14][15][16][17].In response to this issue, the introduction of energy supplements from the external environment presents a promising strategy for energy conversion and storage [18,19].In this way, solar energy, as a clean, abundant and sustainable energy source, has generated wide interest and been adopted to devices for CO 2 reduction or electricity conversion and storage of electrical energy [20][21][22][23][24].
However, the overall impression from the previous works on electrode design of Li-CO 2 batteries is confined to operating only at room temperature.For the practical use of Li-CO 2 batteries in applications, such as mars landing and deep space exploration, low-temperature operation is an essential requirement [25][26][27].The decrease of ambient temperature inevitably leads to increased viscosity of electrolyte, increased charge-transfer resistance at the electrode/electrolyte interface, so that more energy is needed to urge the discharge and charge process [28][29][30][31].The electrolyte for low-temperature Li-CO 2 batteries was replaced by the low-temperature adaptive electrolyte as previous work reported, which limited the application of room temperature [32].In order to adapt to wide temperature environments, the thermal effect of solar energy could assist Li-CO 2 batteries without electrolyte replaced in self-heating to meet the requirements [33].As for photo-energized Li-CO 2 batteries, photoelectric effect efficiently accelerates the reaction kinetics of electrochemical reduction of CO (COER) by leap of photons-excited electrons, and strong photothermal effect enhances visible light absorption and the conversion of solar energy to heat [34][35][36][37].Therefore, photoelectric and photothermal synergistic mechanism of photo-energized cathode can effectively speed up the interfacial charge transfer of low-temperature environments, but stable cycling at low temperatures remains an urgent issue to be addressed.
In this study, we design a photo-energized binder-free Li-CO 2 battery with semiconducting 2H-MoS 2 on carbon nanotube (CNT) conductive substrate (MoS 2 /CNT) as a photocathode to content the requirement of wide temperature range application.Combining DFT calculations and optical properties, tightly integrated MoS 2 /CNT with narrow band gap ensures effective absorption of most visible light and subsequently guarantees abundant generation as well as rapid transfer from MoS 2 to CNT of photo-excited electrons and holes.Sensitive current response and significantly reduced impedance illustrate the efficient ions diffuse and enhanced reaction kinetics contributing to the excellent electrochemical performance.The Li-CO 2 battery with MoS 2 /CNT photocathode upon illumination exhibits a higher discharge voltage platform of 2.95 V and the charge voltage down to 3.27 V, leading to high energy efficiency of 90.2% than 74.9% of non-illuminated battery.Benefiting from complete decomposition of insulated discharge products Li 2 CO 3 , the battery shows robust cycle stability over 120 cycles.Due to the graphene-like two-dimensional structure with high specific surface area, MoS 2 demonstrates excellent photothermal and photoelectric synergistic effects [38,39].At an extremely low temperature of − 30 °C, the battery without electrolyte replaced achieves an ultra-low charge voltage of 3.4 V and maintains high energy efficiency of 86.6% by simultaneously promoting the generation of photo-generated charge carriers and heat under illumination.

Carbon Nanotube Paper Activation Treatment
The CNT paper used in this experiment was activated before loading MoS 2 on it.Cut the CNT paper into 2 cm × 4 cm.First, pour 65%-68% concentrated nitric acid into a beaker.The cut CNT paper was completely immersed in concentrated nitric acid, followed by reflux condensation at 90 °C for 9 h.After the acidification treatment, the CNT paper was removed and rinsed several times with deionized water to remove the nitric acid left on the surface of the CNT paper.Finally, the CNT paper was placed on nickel foam in a vacuum drying oven at 60 °C for 12 h.

MoS 2 /CNT Cathode Preparation
0.121 g Na 2 MoO 4 •2H 2 O and 0.157 g CN 2 H 4 S was dissolved with 20 mL deionized water under stirring for 40 min.Then the solution was transferred to a 30 mL Teflon-lined stainless-steel autoclave.A piece of pre-prepared CNT paper (2 cm × 4 cm) was immersed into the solution and the mixture was sealed and heated in an oven at 200 °C for 24 h.After cooling down to room temperature, the CNT paper was taken out and rinsed with deionized water for several times, followed by being dried in a vacuum oven at 60 °C for 12 h.After heat treatment in a tube furnace at 600 °C for 4 h with a slow ramping rate at 2 °C min -1 , the MoS 2 /CNT compound film was obtained and cut into 1.13 cm 2 disks for use as cathode.

Materials Characterization
The morphologies of samples were characterized by scanning electron microscope (SEM, LYRA3, TESCAN, Czech) equipped with element mapping energy-dispersive spectrometer and TEM (Talos F200X G2, FEI).X-ray diffraction (XRD, X'Pert3 Powder, PAN alytical, Netherlands) was conducted with Cu Kα radiation (λ = 0.154178 nm) at a scanning speed of 5° min -1 between 10° and 80°.Raman spectra were obtained at an excitation wavelength of 532 nm.Both Raman and PL spectra were collected using long focal length spectrometer (1000 M Series, Horiba, USA).X-ray photoelectron spectroscopy (XPS) measurements were achieved by PHI-5000versaprobe (Thermo Fisher Scientific, USA) with Al Ka (1486.6 eV) as the X-ray source.UV-Vis absorption spectrum was achieved by Lambda 1050 + UV/VIS/NIR Spectrometer (PerkinElmer, USA).With the objective of highly effective conductivity carriers and maximized separation of the photo-generated electrons/holes, a tightly integrated and binder-free MoS 2 /CNT photo-electrode was designed and prepared according to a hydrothermal synthesis strategy (Fig. S1) [40].The interior structure of the MoS 2 /CNT was observed via SEM.Compare with the acquired pristine CNT without impurity (Fig. S2), as-synthesized MoS 2 /CNT image (Fig. 1a) exhibits clearly visible MoS 2 nanosheets growing on carbon fibers.The random-orientated MoS 2 intercrossing with each other guarantees a porous and gas-permeable nanostructure with abundant reaction sites.The higher-magnification microscopy image in Fig. 1b demonstrates that tubular carbon fibers were tightly wrapped by MoS 2 nanosheets via hydrothermal and thermal treatment with uniform diameters of around 50-100 nm.The energy-dispersive spectroscopy (EDS) of MoS 2 /CNT reveals that MoS 2 homogeneously distributed on the CNTs without any observable MoS 2 nanoclusters (Fig. 1c).Avoiding the accumulation and agglomeration of bulk MoS 2 , two-dimensional structural photocatalysts growing directly onto the conductive substrate enables not only efficient diffusion of CO 2 but full penetration of electrolyte.Moreover, the adequately contacted heterostructure with high specific surface area provides abundant redox reaction sites and rapid transport of photo-generated carriers.The transmission electron microscopy (TEM) image in Fig. 1d demonstrates that MoS 2 nanosheets uniformly grew along the intercrossed CNTs, which corresponds the SEM and EDS results.As shown in Fig. 1e, the high-magnification TEM image presents the multilayer structural MoS 2 nanosheets with a clear lattice spacing of 0.624 nm, which is consistent with the d-spacing in the (002) direction of 2H-MoS 2 [41].

Photo
The sample identification and crystallographic structure was characterized by XRD.As shown in Fig. 1f, diffraction peaks locate at 14.0°, 33.2°, 39.3°, and 58.8°, respectively, correspond to the values of (002), (100), (103), and (110) crystal planes of hexagonal MoS 2 in the standard card (JPCDS #37-1492) [42].The Raman spectra in Fig. S3 illustrates that the major peaks at 380 and 405 cm -1 are consistent with the in-plane and an out-of-plane typical E 1 2g and A 1g vibrational modes, which confirm the 2H-MoS 2 phase [43].The precise chemical states of Mo and S in MoS 2 /CNT were identified by XPS in Fig. S4.
The characteristic peaks at 232.78 and 229.68 eV are assigned to the emission from the electrons of Mo 3d 3/2 and Mo 3d 5/2 , respectively.In addition to a pair of characteristic peaks observed at 232.78 and 229.68 eV belonging to Mo 4+ , a small peak located at 236.08 eV would be labeled as Mo 6+ 3d 3/2 , and the appearance of Mo 6+ indicates the surface oxidation of molybdenum trioxide [44].The peak detected at 226.78 eV is classified as the S-S bond from the residual sulfur that has not reacted with molybdenum.In the S-2p spectrum, the peaks observed at 163.68 and 162.48 eV are assigned to S 2p 1/2 and S 2p 3/2 , respectively, confirming the above result of the existence and elemental state of MoS 2 [45].Density functional theory (DFT) calculations were carried out to get insight into the electronic behaviors of MoS 2 /CNT.The electron density of states was performed to show the electron-rich regions at the S units of MoS 2 and the electron-depletion regions at the CNT part, implying the electron spontaneous redistribution from MoS 2 to CNT (Fig. 2a) [46,47].Moreover, the density of states (DOS) and partial density of states (PDOS) of was calculated to analyze the interfacial electronic structures of MoS 2 [48].The DOS results further demonstrate equal amount of spin-up and spin-down electrons, illustrating the structural stability of electrons in pristine MoS 2 and MoS 2 / CNT.Different from the DOS of pristine MoS 2 structure, two new peaks from 0.85 to 0.02 eV in p orbit appear near Fermi level, implying that the introduction of CNT boost electronic migration via enabling more available electron states near Fermi level for MoS 2 (Fig. 2b, c).The PDOS of pristine single-layer MoS 2 and MoS 2 /CNT composite in Fig. S5 illustrates that the valance band maximum is contributed by the d orbit of Mo and p orbit of S, while conductive band minimum is dominated by S-3p state.In this state, Fermi energy level located in the interval of zero value without passing-through electron state and the electron state near Fermi level is primarily composed of Mo-4d, indicating the semiconducting properties of pristine MoS 2 and MoS 2 /CNT.Both pristine MoS 2 and MoS 2 combining with CNT are primarily contributed by Mo-4d state and S-3p state.From 7 to 1 eV of the VB, 4d orbit of Mo overlap with 3p orbit of S, implying the presence of orbital hybridization in MoS 2 [49].
To investigate the effect of light on photo-electrode, the optical properties including light-harvesting ability, band structure and photo-generated carriers separation efficiency were further analyzed.Figure 2d shows the UV-Vis absorption in the range of 300-800 nm, in which MoS 2 /CNT demonstrates stronger absorption intensity and distinct absorption peak compared with CNT.In order to evaluate the band gap, Tauc plot (Fig. S6) corresponding to the UV-Vis absorption spectrum was carried out, thus deriving an optical energy gap of 1.25 eV, which is consistent with above results of multilayer 2H-MoS 2 .As shown in Fig. 2e, the positive slope in the Mott-Schottky (M-S) plot illustrates the n-type semiconducting nature of the MoS 2 /CNT cathode and an estimated flat band potential of 2.50 V versus Li + /Li which is more positive by about 0.1 V than the CB.Combining the valves of band gap and CB, the VB edges of MoS 2 /CNT is calculated to be 3.74 V versus Li + /Li.As for the generation of photogenerated carriers, the photoluminescence spectroscopy (PL) images show that compared with CNT, MoS 2 /CNT was observed with obvious PL peaks, indicating that the presence of MoS 2 generate electron/hole pairs on cathode under illumination (Fig. 2f).The fluorescence lifetime spectra results illustrate that the interaction of MoS 2 /CNT prolongs the fluorescence lifetime of photo-generated carriers from MoS 2 , which provides evidence for more efficient electron transfer attributed to the excellent electrical conductivity of CNT (Fig. S7).As shown in Fig. 2g, the MoS 2 /CNT photo-electrode satisfies the basic conditions for light-promoted Li-CO 2 batteries: the potential of the evolution of CO 2 and Li 2 CO 3 /C (2.80 V vs Li + /Li) lies between the CB and VB potentials of photocathode.We conducted first principles calculations using DFT to investigate the dynamic processes under both light and non-light conditions (Tables S1, S2 and Fig. S8).Reaction a1 is the rate-determining step during the charging process, and the Gibbs free energy of this reaction is 4.6250 eV in the absence of light.After applying light, the energy barrier decreases by 0.2965 eV.During the

Electrochemical Properties of MoS 2 /CNT Photocathode
The photo-energized Li-CO 2 battery was assembled with a MoS 2 /CNT photocathode, a lithium anode.A Xe lamp with a power density of about 100 mW cm -2 was utilized as light source, providing photoenergy with a wavelength range from 380 to 780 nm.CV curves of Li-CO 2 batteries with MoS 2 / CNT cathode in Ar or CO 2 depicts significantly larger area  In order to further investigate the effect of illumination, LSV of Li-CO 2 batteries was evaluated.Under illumination, MoS 2 /CNT exhibits a more pronounced current density than that under no illumination in reduction process, indicating its enhanced dynamic kinetics and conductivity during discharge/charge process (Figs.3c and S10a).The Tafel slope inferred from the LSV data demonstrates that the photoassisted value of 29.84 mV dec -1 is much smaller than that under no illumination (164.97 mV dec -1 ) (Fig. 3d).For the opposite oxidation process, a similar result is observed that the slope with illumination is smaller than that in the dark, confirming better oxidation kinetics ascribed to the contribution of photo-generated carriers (Fig. S9b).
The EIS was implemented to further evaluate ion transport properties of the Li-CO 2 battery with the effect of illumination.Consistently, the plot of EIS in Fig. 3e shows a much smaller impedance of illuminated MoS 2 /CNT cathode than that under no illumination, which illustrates the rapid ions diffuse in battery.Galvanostatic intermittent titration tests (GITT) during discharge and charge was performed to further explore the positive effect of solar energy on the catalytic performance of the photocathode.During the discharge process, the overpotential (0.25 V) of the photo-energized battery is significantly lower than that of around 0.47 V upon the dark condition, implying the compensation of the internal generated photovoltage for the high overpotential in the light-treated Li-CO 2 battery (Fig. 3f).The reverse charging process also exhibits consistent lower overpotential, revealing that abundant photogenerated electrons from photocathode facilitate the evolution of insulating discharge products (Fig. S11).Above results suggest that solar energy is converted into electrical energy storage during discharge and compensates for the high potential required for product decomposition, which promotes the intrinsic kinetics of Li-CO 2 battery.

Reversibility of CORR/COER and Analysis of the Discharge Products
The kinetic factors that vary with photoenergy affect the formation and decomposition of discharge product.To in-depth evaluate the effect of illumination on the morphology evolution of reaction product, the MoS 2 /CNT photocathodes after discharge and recharge at 0.05 mAh cm -2 with and without illumination were analyzed via SEM.Bulk discharge products formed by particles deposit on the surface of discharged MoS 2 /CNT cathode without illumination (Fig. 4b) and remain a small amount of residual after recharge with the same capacity (Fig. 4d).In sharp contrast, the light-mediated products exhibit film-like morphology which are mostly decomposed on recharged MoS 2 /CNT cathode.And a nearly clean cathode surface is delivered during the recharging process, suggesting the efficient catalytic performance of photo-generated carriers promote reversible decomposition of products (Fig. 4a, c).XRD characterization of MoS 2 /CNT was carried out to analyze the composition and evolution of the discharge products (Fig. 4e).Three characteristic peaks of Li 2 CO 3 (2θ = 21.28°,30.58°, and 31.68°)appear after discharge upon both illumination and no illumination [52].
After recharging process, the peaks corresponding to Li 2 CO 3 disappear, confirming the reversible reaction in the Li-CO 2 battery.To further measure the reversibility of Li-CO 2 battery, differential quantitative mass spectrometry (DEMS) was performed to evaluate the CO 2 conversion during the discharge and charge processes under a constant current density of 0.1 mA (Fig. S12) [53].A discharge or charge capacity of 0.1 mAh corresponds to a theoretical CO 2 evolution of 2.8 µmol.Under illumination, the consumption and release of CO 2 were 2.37 and 2.07 µmol.Correspondingly, in the absence of light, the CO 2 conversions during the discharge and charge processes were 2.20 and 1.54 µmol, respectively, indicating the superior ability of photoenergy on boosting decomposition of Li 2 CO 3 and the reversibility of reaction.
A feasible mechanism for the tremendous difference in discharge product morphology is schematically clarified in Fig. 4f.Under illumination above the band gap energy, a large number of photoelectrons are excited from VB to CB in MoS 2 , delivering abundant available active sites for nucleation.Benefited from ample nucleation sites and fast-diffusing Li + , the Li 2 CO 3 on photocathode grows more dispersive on surface and much slower in size than that in the dark.Therefore, Li 2 CO 3 exhibited as thin film under light after discharge.During charging, sufficient photo-generated holes and better electronic transport of film-like morphology contribute to the decomposition of discharge Li 2 CO 3 , enabling the charge process at a much lower overpotential.On the contrary, bulk Li 2 CO 3 formed by the accumulated particles grows on discharging cathode surface without illumination owing to slow Li + spread and few nucleation sites.In the reverse charging process, bulk Li 2 CO 3 decomposes difficultly due to the sluggish reaction kinetics caused by the absence of light, resulting in high overpotential.As cycle number increases, the accumulation of incompletely decomposing Li 2 CO 3 hinders CO 2 permeation and requires more energy for oxidation, following with higher and higher voltage gap and the loss of electrochemical performance.

Electrochemical Performance of Room Temperature Li-CO 2 Batteries
After confirming the photoelectric effect of the MoS 2 /CNT photocathode in electrochemical kinetics, the electrochemical properties of Li-CO 2 battery were systematically evaluated.The discharge and charge profiles of at various current densities were performed in Figs.5a and S13.Even with compensation of the photovoltage, similar to light-free condition, the polarization under illumination increased along with the current density, owning to the limitation of finite photo-generated carriers.At 0.02 mA cm -2 , the overpotential under illumination rise to 0.55 V, which is 0.62 V lower than that of the battery without illumination.Similarly, owing to the compensative current, the polarization upon illumination increases along with the current density and rise to 0.91 V at 0.05 mA cm -2 , which is 1.04 V lower than that of non-illuminated battery.As the current increases to 0.05, 0.10, 0.20, and 0.50 mA cm -2 , the light-mitigated overpotential are 0.82, 1.11, 1.31, and 1.95 V, respectively.In sharp contrast, the charge voltage in the dark at 0.05 mA cm -2 hit the voltage cutoffs of 5 V, resulting in an extremely high voltage gap up to 2.64 V.With the increasing current density, Li-CO 2 battery without illumination Meanwhile, fully discharged or charged with cut-off voltages of 2 or 4 V, the photo-energized battery provides high area capacities of 4.88 and 4.21 mAh cm −2 , respectively, while the corresponding capacities of non-illuminated batteries are only 0.40 and 0.10 mAh cm -2 (Fig. 5b).The significant capacity increase is due to the great promotion on discharge performance of film-like Li 2 CO 3 upon illumination.While the stacked bulk Li 2 CO 3 is observed on completely discharged cathode without illumination, which is more difficult to decompose than that the film-like Li 2 CO 3 , as shown in Fig. S14.Furthermore, the photo-responsive voltage visually demonstrates the effect of solar energy on the potential during the discharge/charge process.As shown in Fig. 5c, the photo-responsive discharge voltage of the Li-CO 2 battery with MoS 2 /CNT cathode rise from 2.68 to 2.90 V, and the charge voltage of 3.78 V rapidly decreases to 3.54 V.The sensitive and efficient photo-responsive voltage implies the sufficient generation and easy transport of photo-generated carriers in the MoS 2 /CNT cathode.
Figure 5d, e depicts the rate capability and corresponding average terminal potential at different current densities from 0.01 to 0.5 mA cm -2 at a fixed capacity of 0.1 mAh cm -2 .In the entire current density range, the discharge voltages of the photo-treated cathode are higher than those in the dark and the charge voltages keep lower than the corresponding voltages of the non-illuminated cathode.Figure 5e visualizes the enhancement of overpotential gap in the dark or light with increasing current density during charging.When the current density is reduced to 0.01 mA cm -2 , the voltage recovers to a value similar to that of the first five cycles, revealing the excellent reversibility inside the battery.The cycling performance of the battery was measured by galvanostatic discharge/charge at a current density of 0.02 mA cm -2 .As shown in Fig. 5f, the light-mitigated discharge terminal voltage per cycle was consistently higher than that of the non-illuminated battery, resulting in the retention of lower overpotential and higher energy efficiency.After 120 cycles, the efficiency under light remains at 85% and polarization is very low (Fig. 5g).As shown in Fig. S15, the overall performance of the MoS 2 /CNT-based battery is superior to that of the previously reported photoassisted Li-O 2 and Li-CO 2 batteries [54][55][56][57][58][59].The efficiency reduction upon cycling is inferred to the accumulation of the volatilization of electrolyte during the long-term operation of the battery [60].Correspondingly, the disparity in electrochemical properties can be well explained by the differences in kinetics and product morphology [61,62].
The photo-enhanced ion transport, electron conduction, current density and active sites boost the formation and decomposition of discharge products, which are further manifested by superior overpotential, capacity and cycling performance in electrochemical properties.

Electrochemical Performance of Low-Temperature Li-CO 2 Batteries
Based on the above results, the photoelectric effect of MoS 2 effectively improves the electrochemical performance of room temperature Li-CO 2 batteries, which has outdistanced the reported work [50,63].However, as shown in Fig. S16, the ionic conductivity of conventional electrolytes decreases drastically with decreasing temperature, which makes the kinetics of battery more sluggish at low temperatures.More importantly, photothermal effect of MoS 2 can achieve extreme low-temperature Li-CO 2 batteries by conversion of solar energy to heat.The low-temperature batteries were assembled in low-temperature control device without electrolyte replaced.To evaluate the photothermal conversion of the MoS 2 /CNT cathode under extreme conditions of − 30 °C, an infrared thermal imager (IR) was used to monitor the temperature of the cathode in real-time.By irradiating the electrode with a light source, the IR images are shown in Fig. 6a, it can be seen that MoS To further explain the photoelectric and photothermal synergistic effects of MoS 2 /CNT at low temperatures, we compared the on/off light current response and up/ cool thermal current response as shown in Fig. 6b.In the thermal current response curve (green line), the current density response delayed by 43 s after thermal excitation, and then reach the maximum platform after about 300 s.While MoS 2 /CNT warms from − 30 to − 12 °C within 10 s without illumination, the hysteresis of thermal response is limited by the heat conduction process.For the on/off photocurrent response of − 30 °C, the response of increased current density appears immediately within 1 s and quickly reaches the maximum platform, which indicating that photoelectric effect is far more sensitive than thermal response on the MoS 2 /CNT cathode.In addition, the low-temperature Li-CO 2 battery shows a higher current platform than thermal effect, indicating that the MoS 2 /CNT in the presence of illumination has the photoelectric and photothermal synergistic, which makes the photo-energized Li-CO 2 battery has catalytic performance than the thermal effect of environmental heating alone.
To confirm this assumption, we assembled a photoenergized Li-CO 2 battery employing same electrolyte and Li foil as room temperature batteries, and compare electrochemical performance under three conditions with illumination at − 30 °C, without illumination at − 30 °C, and without illumination at − 12 °C.As shown in Fig. 6c, due to the slow reaction kinetics caused by low temperature in the − 30 °C, the Li-CO 2 battery without illumination exhibits a low final discharge voltage of 2.18 V and a high final charge voltage of 4.22 V, resulting in an ultra-high over gap of 2.04 V.Under the ambient temperature of − 12 °C, (2025) 17:5 5 Page 12 of 16 https://doi.org/10.1007/s40820-024-01506-1© The authors the Li-CO 2 battery without illumination exhibits higher the final discharge voltage of 2.58 V and lower charge voltage of 4.05 V, which indicate temperature dependence of reaction kinetics.The Li-CO 2 battery with illumination at -30 °C further increases the final discharge voltage to 2.78 V and reduces the final charge voltage to 3.60 V, demonstrating a superior photothermal and photoelectric synergistic enhancement effect compared to the thermal effect under isothermal conditions.Figure S18 indicates that conventional electrolytes can also operate efficiently at low temperatures in the presence of light.The CV curves were further measured for the Li-CO 2 battery to explore the catalytic performance at different temperature conditions.As is evident from the Fig. S19, the battery working at − 30 °C under illumination presents a lower onset evolution potential as well as a higher onset reduction potential along with significantly larger currents compared to the battery working at − 12 and − 30 °C without illumination.Moreover, Fig. S20 shows the discharge/charge behaviors responding to the Li-CO 2 battery with illumination at − 30 °C and without illumination at − 12 °C.When the working environment is changed from illumination to only heating at − 12 °C, there is a rapid decrease in the discharge voltage, along with a quick increase of the charge voltage.This suggests both the discharge and charge processes of the Li-CO 2 battery will be promoted by the photo-generated carriers.
Figure 6d shows the EIS of the Li-CO 2 battery with illumination at − 30 °C, without illumination at − 30 °C, and without illumination at − 12 °C.The plot of EIS with illumination at − 30 shows a much smaller impedance of illuminated MoS 2 /CNT cathode than that under without illumination at − 12 and − 30 °C, which illustrates that photothermal and photoelectric synergistic accelerate the rapid ions diffuse and reaction kinetics at low temperature.In order to further investigate the effect of illumination, LSV curves of Li-CO batteries was evaluated.Under illumination at − 30 °C, the onset potentials for MoS 2 /CNT were found to be higher than those without illumination at − 12 and − 30 °C.For the CO 2 reduction reaction in Fig. 6e, MoS 2 /CNT exhibits a higher current density than that under no illumination.And the Tafel slope inferred from the LSV data demonstrates that the photo-assisted value of 16.42 mV dec −1 is much smaller than that under no illumination at − 12 °C (61.96 mV dec −1 ) and −30 °C (181.76 mV dec -1 ) (Fig. 6f).For the opposite oxidation reaction, a similar result is observed in Fig. S21.The highest current density and the smallest Tafel slope with illumination at − 30 °C are attributed to enhanced dynamic kinetics and conductivity during discharge and charge process.As a result, the Li-CO 2 battery under illumination at − 30 °C shows smaller over gap during 10 cycles compared to the battery without illumination at − 12 °C as shown in Fig. S22, indicating that the photo-energized effect can superimpose the photoelectric effect on the thermal effect, further improving the cycling performance of low-temperature Li-CO 2 batteries.Besides, the rate capability of the Li-CO 2 battery under illumination at − 30 °C in Fig. S23 exhibits smaller polarization with increasing current density than the battery without illumination at − 12 °C.

Conclusions
In summary, we successfully developed a photo-energized Li-CO 2 battery based on MoS 2 /CNT photo-electrode as cathode in wide temperature range application.The binderfree structure of MoS 2 /CNT cathode enables abundant generation and rapid transfer of photo-excited carriers, which facilitates the intrinsic dynamic kinetics.Upon illumination, photo-generated electrons transiting from VB to CB migrate to participate in the reduction of CO 2 , leading to different morphology during discharge process.During reverse charge process, photo-generated holes have a favorable impact on the decomposition of insulated discharge products Li 2 CO 3 .Consequently, the photo-energized room temperature battery exhibits a higher discharge voltage platform of 2.95 V and the charge voltage down to 3.27 V, leading to high energy efficiency of 90.2% than 74.9% of non-illuminated battery.And excellent cycling stability indicates the conversion and compensation of photoenergy for electrochemical reaction.
Toward extreme low temperature, the highly performance Li-CO 2 batteries profit from the photoelectric and photothermal synergistic mechanism of MoS 2 /CNT cathode, achieving an ultra-low median charge voltage of 3.4 V at − 30 °C with a round-trip efficiency of 86.6%.These results propose useful guidelines for MoS 2 as photocathode in performance enhancement of photo-energized Li-CO 2 systems in a wide temperature range for energy storage.

Fig. 1 a
Fig. 1 a, b Scanning electron microscopy images of MoS 2 /CNT.c Energy-dispersive spectroscopy images of C, Mo, and S of MoS 2 /CNT.d, e Transmission electron microscopy images of MoS 2 /CNT.f X-ray diffraction patterns of the MoS 2 /CNT cathode /doi.org/10.1007/s40820-024-01506-1© The authors discharge process, reaction b5 is the rate-determining step, and the Gibbs free energy under non-light conditions is 6.4143 eV.After applying light, the energy barrier decreased by 0.1324 eV.The above results present that the effective generation, separation and transfer of photo-generated electrons/holes on MoS 2 /CNT cathode, implying promising photo-energized electrochemical performance during the reaction in Li-CO 2 battery for energy conversion and storage[50,51].

Fig. 2 a
Fig. 2 a Charge density plot of MoS 2 /CNT.Density of States plot of b MoS 2 and c MoS 2 /CNT.d UV-Vis absorption spectra of MoS 2 /CNT and CNT. e Mott-Schottky spectra of MoS 2 /CNT.f Photoluminescence spectroscopy spectra of CNT and MoS 2 /CNT.g Working mechanism and energy levels of the photo-energized Li-CO 2 battery based on the MoS 2 /CNT cathode

Fig. 3 a
Fig. 3 a First discharge and charge curves of the Li-CO 2 battery based on the MoS 2 /CNT cathode with and without illumination at 0.01 mA cm -2 .b Photocurrent response of of MoS 2 and MoS 2 /CNT.c Linear sweep voltammetry curves in CO 2 reduction process at 5 mV s -1 and d corresponding Tafel curves, e electrochemical impedance spectroscopy spectra, f Galvanostatic intermittent titration spectra during discharge of Li-CO 2 battery with MoS 2 /CNT cathode in the presence and absence of illumination

Fig. 4 a
Fig. 4 a-d Scanning electron microscopy images and e X-ray diffraction patterns of the MoS 2 /CNT cathodes collected from Li-CO 2 batteries at corresponding discharge/charge states.L-Dis, L-Cha, D-Dis, and D-Cha represent the batteries discharged or recharged at 0.05 mAh cm -2 in the light or dark.f Model illustrating the effect of illumination on deposition and decomposition of discharge products in Li-CO 2 batteries

Fig. 5 a
Fig. 5 a Discharge and charge curves of the Li-CO 2 battery based on the MoS 2 /CNT cathode with illumination at different current density.b Full discharge/charge profiles with cut-off voltages of 2 and 4 V. c Photo response of the discharge/charge voltage at a current density 0.01 mA cm -2 switching between "on" and "off".d Rate capability, e average charge terminal voltage at different current density, f cycling profiles at 0.02 mA cm -1 of Li-CO 2 battery with MoS 2 /CNT cathode in the presence and absence of illumination, g energy efficiency and voltage gap of the battery

Fig. 6 a
Fig. 6 a IR images of top sides of MoS 2 /CNT cathode with illumination at − 30 °C.b On/off current response of MoS 2 /CNT under lighting (blue) and heating (orange) conditions at − 30 °C.c First discharge and charge curves, d electrochemical impedance spectroscopy spectra, e linear sweep voltammetry curves in CO 2 reduction process at 5 mV s -1 , and f corresponding Tafel curves of Li-CO 2 battery with MoS 2 /CNT cathode with illumination at − 30 °C, without illumination at − 30 °C, and without illumination at − 12 °C

3 Result and Discussion 3.1 Synthesis and Analysis of Binder-Free MoS 2 /CNT Photo-Electrode
2 /CNT gradually heats up with increasing irradiation time.After 50 min, the center temperature rises from − 30 to − 12 °C.Due to the continuous cooling of liquid nitrogen, there is no significant increase in temperature in the later stage.The IR image indicates the photothermal effect of MoS 2 /CNT under lowtemperature conditions upon illumination, resulting in a temperature difference of approximately 18 °C.Based on the data obtained from IR images, the temperature gradient of the battery under − 30 °C illumination can be obtained.As shown in the Fig.S17, considering the thermal conductivity of the battery, the photo-energized low-temperature Li-CO 2 battery based on MoS 2 /CNT cathode exhibits a temperature gradient of cathode-electrolyte-anode from − 12 to − 30 °C in this experimental environment.